The present invention relates to thermal transfer printing, and, more specifically, to multi-use transfer elements suitable for use in thermal transfer printing processes. One embodiment of the present invention resides in a process for forming images which comprises (a) incorporating into a thermal transfer printing apparatus a multi-use thermal transfer element comprising a substrate upon which is situated a porous sponge layer filled with ink; (b) applying heat imagewise to a portion of the substrate to heat the ink contained in the porous sponge layer, thereby enabling transfer of the ink in imagewise fashion from the porous sponge layer to a receiver sheet in contact with the porous sponge layer; (c) thereafter separating the receiver sheet of step (b) from the transfer element; (d) subsequently contacting the porous layer situated on the portion of the substrate heated in step (b) with a receiver sheet; and (e) thereafter applying heat imagewise to the portion of the substrate previously heated in step (b) to heat the ink contained in the porous sponge layer, thereby enabling transfer of the ink in imagewise fashion from the porous sponge layer to the receiver sheet of step (d). Another embodiment of the present invention employs resistive heating to generate images by applying voltage imagewise to the substrate of the transfer element to transfer ink to the receiver sheet.
Thermal printing is a nonimpact printing process that enables formation of high resolution images. These printing processes are simple, offer low noise levels, and are very reliable over extended usages. Thermal printing processes can be classified into three categories. Direct thermal printing entails the imagewise heating of special papers coated with heat sensitive dyes, such that an image forms in the heated areas. Another method of thermal printing is known as the dye transfer or dye sublimation technique, and operates by heating a transfer element coated with a sublimable dye. When the transfer element is imagewise heated, the dye sublimates and migrates to the receiver sheet, which possesses a polymeric coating into which the dye diffuses, forming an image. A third method of thermal printing is known as thermal transfer printing. The thermal transfer printing process entails imagewise heating of a transfer element containing ink, which transfer element is in intimate contact with the heater on one side and with the receiving substrate on the other side. Imagewise heating of the transfer element affects the ink in such a way as to cause it to transfer from the transfer element to the receiving substrate, thereby resulting in image formation. Thermal transfer printing methods generally employ uncoated plain papers, which enables prints with acceptable appearance and excellent archival properties. In addition, the thermal transfer printing method can be employed for color printing applications by using transfer elements of the desired color or colors.
Thermal transfer printing processes generally employ a thermal printhead, a transfer element, and a receiver sheet. The side of the transfer element containing the ink is placed in contact with the receiver sheet, and heat originating from the printhead is then applied to the transfer element. Heat conducted through the transfer element increases the temperature of the ink, which can cause it to melt, soften, decrease in viscosity, or otherwise undergo a transition that enables at least some of the ink to transfer to the receiver sheet. After the receiver sheet and transfer element are separated, an image remains on the receiver sheet. An alternative method of heating the transfer element, known as resistive heating, employs an array of electrodes instead of thermal printhead to generate a current between the electrodes and a grounded conductive layer in the transfer element. This method is described in the IBM Journal of Research & Development, Vol. 29, No. 5, 1985, the disclosure of which is totally incorporated herein by reference. Additional information concerning thermal transfer printing processes is disclosed in Thermal Transfer Printing: Technology, Products, Prospects, published by Datek Information Services, P.O. Box 68, Newtonville, Mass., the disclosure of which is totally incorporated herein by reference.
The thermal transfer printing process has been disclosed in, for example, U.S. Pat. No. 3,441,940 and U.S. Pat. No. 3,745,586, the disclosures of each of which are totally incorporated herein by reference. In addition, augmented thermal transfer printing processes are known. For example, U.S. Pat. No. 3,989,131 discloses a pressure assisted thermal transfer printing process employing an electrothermic printing unit for writing dot matrix characters on a printing line of recording medium by means of an electrothermal printing head which is continually movable along the printing line. Pressure is interposed between the head and the recording medium, pressure means being provided for pressing the printing elements against the transfer element and the receiver sheet. In addition, U.S. Pat. No. 4,541,042 discloses a transfer recording process assisted by a solvent, wherein a receiving medium such as paper and an ink transfer sheet are placed in contact between a platen and a thermal head, and a liquid, volatile solvent is applied to the paper. The solvent enables high speed thermodissolving transfer of the ink to the paper by heating selected areas to form an image.
Further, U.S. Pat. No. 4,525,722 discloses a thermal transfer printing process assisted by chemical heat amplification, wherein some of the heat necessary for melting and transferring the ink from a solid fusible layer in a ribbon to a receiving medium is provided by an exothermic reaction involving an exothermic material contained in a layer in the ink ribbon. Also, U.S. Pat. No. 4,549,824 discloses a thermal transfer printing process aided by an exothermic reaction, wherein an aromatic azido compound is added to the ink, said azido compound being one that exotherms at the conditions of thermal ink transfer. In addition, U.S. Pat. No. 4,550,324 discloses an ink transfer thermal printer utilizing a thermosensitive ink that is solid at normal temperatures, with selected portions of the ink being liquefied by heating and transferred onto recording paper. The printer may be of either contact or non-contact (ink jet) configuration, and eliminates the need to utilize disposable materials such as ink ribbons.
U.S. Pat. No. 4,567,489, discloses a thermal printhead for a thermographic printer having an electrically insulating substrate on which resistors are placed that form impression points and current supply and current discharge leads bonded to the resistors. The printhead includes a structure for forming a magnetic field that acts on the resistors in the immediate proximity of the resistors and along the resistor print line. The magnetic field is directed such that when the current flows through the resistors, the current paths are deflected upward into the upper part of the resistor on its outer surface. The single resistor impression points thus reach their highest temperature at the printing surface where they must deliver heat to the recording medium, which results in the heat needed for heating the resistor being supplied more quickly to the recording medium, thereby reducing the cooling time of the single resistor impression point so that a higher printing velocity can be attained with the thermal printhead.
Additionally, U.S. Pat. No. 4,510,511 discloses a picture recording method and apparatus using an ink containing an evaporable coloring matter, which enables printing on a medium without an ink ribbon. The special ink is supplied to an ink transporting means and then cooled below the melting point of the ink bonding agent. A discharge energy is applied, controlled according to the picture to be formed, which causes the coloring matter to fly to the recording medium opposite the transporting means. Essentially, the process entails fluidizing a marking material by heat, picking up the liquid marking material on a gravure type roll, and selectively transferring it to the receiving sheet by means of a high voltage field.
One difficulty encountered with thermal transfer processes that employ single use transfer elements or is the cost incurred for materials and supplies required for the thermal transfer printing process. A single use thermal transfer element is bulky and expensive since a full panel of the ribbon must be used for each print formed; for full color prints, four panels of a single use transfer element must be used to form a single print. Thus, a thermal transfer element capable of multiple uses greatly reduces the expenses incurred for materials and supplies in thermal transfer printing processes, and also enables ribbon cassettes of smaller size. In addition, a single use thermal transfer element which uses a full panel of the ribbon for each print formed can be readable subsequent to use. A multi-use thermal transfer element, in contrast, offers an information security advantage, since it is generally unreadable subsequent to printing, particularly if the transfer element has been used more than once.
The present invention resides in a multiple use thermal transfer sheet comprising a porous sponge structure from which ink is metered in a controlled fashion. Porous ink bearing media are known. For example, U.S. Pat. No. 4,046,073, the disclosure of which is totally incorporated herein by reference, discloses a printing or copying system in which ink is transferred from an ink-bearing medium, which can be a porous medium filled with ink in the pores, to a printing medium through the use of ultrasonics. The ink bearing medium is placed in contact with the paper and ultrasonic energy is applied to the medium, causing the ink to decrease in viscosity due to ultrasonic vibrations and conversion of ultrasonic energy into heat. The ink is then transferred to a printing medium. In addition, European Patent Application 0,254,420 discloses a recording method wherein a recording member generally in the form of a cylinder and having on its outer surface one or more porous layers impregnated with ink is heated on the outer surface in image configuration. Ink contained in the cylinder is thus brought to the surface of the imaging member and transferred to a recording sheet in imagewise fashion.
Ink compositions containing liquid crystalline materials are known. For example, U.S. Pat. Nos. 3,969,254 and 4,022,706 disclose cholesteric liquid crystal water base inks. Films formed from these inks can be used as temperature indicators and ornamental articles, and may also be used for photograph reproduction by irradiating through a negative placed on the film such that the exposed areas of the film undergo a temperature response change which is stable for long periods of time. Typically, the inks comprise an oil in water latex, a small amount of organic solvent, which is usually polar and moderately water soluble, liquid crystals, a thickening agent, and, optionally, a wetting agent.
In addition, U.S. Pat. No. 3,974,317 discloses thermometric compositions for recording changes in temperature which comprise a cholesteric liquid crystal system and a chemically inert substance immiscible with the crystal system. Varying the amount of the inert substance in the composition varies the temperature at which a predictable phase change to the isotropic phase occurs. All compositions in a particular product have the same color and exhibit the same color change.
Further, U.S. Pat. No. 3,666,947 discloses a liquid crystal imaging system employing an imaging member with a composition having a cholesteric liquid crystalline phase with a radiation absorptive material dispersed throughout the liquid crystalline material. The imaging member is thermally imaged by heating portions of the material. The imaging members may also be erased by application of external forces, such as electric or magnetic fields.
In addition, U.S. Pat. No. 4,803,119, the disclosure of which is totally incorporated herein by reference, discloses ink coating compositions for impact typewriter ribbons, which ink coatings comprise a sponge material having dispersed therein an ink comprising pigment particles and a dimer acid. Further, U.S. Pat. No. 3,348,651, the disclosure of which is totally incorporated herein by reference, discloses pressure sensitive ink transfer ribbons, tapes, and sheets having a microporous inking composition for use in typewriters, high speed printers, and optical scanning devices. The pressure sensitive ink transfer medium comprises a shock-absorbent base layer of an elastomeric polymer film having a high degree of resiliency in a direction normal to the plane of the film, an intermediate layer of a thin, non-elastic polymer film bonded to the base layer, and an inking layer bonded to the intermediate layer over substantially its entire working surface and comprising a substantially continuous film of a microporous inking composition. The microporous inking composition consists essentially of a uniformly blended mixture of an elastomeric polymeric binder, an inking compound comprising a non-aqueous, non-volatile ink carrier which is substantially insoluble in the elastomeric polymeric binder and which contains a high concentration of an ink pigment, and a finely ground microporous inorganic filler. Other patents, such as U.S. Pat. No. 3,287,153, U.S. Pat. No. 3,392,042, U.S. Pat. No. 3,484,508, U.S. Pat. No. 3,930,099, U.S. Pat. No. 4,321,286, U.S. Pat. No. 4,544,292, and U.S. Pat. No. 4,624,881, also disclose pressure sensitive porous marking ribbons filled with an exudable marking material. In addition, U.S. Pat. No. 3,351,948, U.S. Pat. No. 3,847,265, U.S. Pat. No. 4,251,276, U.S. Pat. No. 4,414,555, U.S. Pat. No. 4,415,903, U.S. Pat. No. 4,603,986, U.S. Pat. No. 4,608,577, U.S. Pat. No. 4,762,734, U.S. Pat. No. 3,480,962, U.S. Pat. No. 4,128,345, U.S. Pat. No. 4,205,320, U.S. Pat. No. 4,315,267 are of collateral interest.
Although the prior art transfer elements are suitable for their intended purposes, a need continues to exist for thermal transfer elements. In addition, there is a need for thermal transfer elements capable of multiple uses. A need also exists for thermal transfer elements which reduce the cost of thermal transfer supplies in a printing device. Further, there is a need for multi-use thermal transfer elements from which a thermally transferrable ink is metered in a controlled fashion during the printing process. A need also exists for multi-use thermal transfer elements capable of forming images on plain uncoated papers. In addition, there is a need for thermal transfer printing elements which cannot be read subsequent to use. There is also a need for thermal transfer elements from which a greater number of prints can be obtained, which enables smaller ribbon cartridges and reduces the storage space necessary for the ribbons. Further, a need exists for thermal transfer elements capable of forming images on rough papers. Additionally, a need exists for multi-use thermal transfer elements suitable for use in thermal transfer printing systems employing resistive heating.